The invention relates generally to DC to ac inverters. More specifically, the invention relates to methods and systems for multilevel inverters having a simplified topology.
Today, the power industry has revived and entered a new age using renewable energy, and high efficiency power generation, transmission and distribution where multilevel power converters can assume significant roles.
Multilevel power converters emerged from the realization that single power semiconductors cannot meet the voltage requirements required for medium voltage power conversion. Multilevel power conversion has rapidly grown in the field of power engineering for the applications of medium voltage ac drive, Flexible AC Transmission System (FACTS) devices, Medium Voltage DC (MVDC) transmission, and High Voltage DC (HVDC) transmission systems. Despite today's high power semiconductor technology that has reached 6.5 kV and 2.5 kA power ratings, multilevel power converters built with mature medium power semiconductor technology present competitive alternative solutions with many advantages over conventional 2-level converters due to their ability to synthesize waveforms exhibiting lower distortion and better harmonic cancellation, smaller
  dv  dtand common-mode voltage, and low switching frequency operation, and attain higher voltages using semiconductor devices with smaller voltage ratings.
Topologically, a multilevel structure can be considered as an ac voltage synthesizer realized from multiple discrete DC voltage sources. Multiple, equal DC sources are required. Multilevel inverters provide an ac output waveform at discrete voltage levels. The more steps or levels generate a smoother sinusoidal waveform and reduce the amount of output filtering. Practically, it is a trade-off to select the number of levels considering the converter complexity and filter requirements. By optimizing the angles and heights of steps, certain lower order harmonics can be cancelled. In addition, the harmonics spectrum can be reduced by using Pulse Width Modulation (PWM) techniques at each level.
Numerous multilevel inverter topologies have been proposed and studied for power utility and motor drive applications. FIGS. 1A and 1B show prior art half-bridge and full-bridge diode-clamped or neutral-point-clamped (NPC) inverters. FIGS. 2A and 2B show prior art half-bridge and full-bridge capacitor-clamped or flying-capacitor inverters. FIG. 3 shows a prior art cascaded H-bridge inverter with separate dc supplies and an ac output at nodes A and B. FIG. 4 shows a prior art topology that replaces an H-bridge architecture with cascaded 2-terminal submodules without separate dc sources to form a modular structure.
The diode-clamped multilevel converter is the most widely used inverter topology. However, this topology is cumbersome to implement for levels beyond five.
There is a need for a simplified inverter topology that allows for levels greater than five.